Document U.S. Pat. No. 5 295 732 describes a seat having a seat back which has:
a front face facing towards the user; PA1 a rear face facing in the opposite direction to the front face and a portion of which is re-entrant to form a cavity in that rear face; PA1 a seat back structure comprising at least two lateral uprights; PA1 upholstery which extends across the front face of the seat and the lateral uprights; PA1 a cover covering the upholstery; and PA1 a sheet disposed in said cavity between the two lateral uprights. PA1 the lateral uprights each comprise at least one web and one rim, the rim being bent to define with the web a housing, each web having an inside face facing towards the inside of the seat; PA1 the upholstery additionally extends over the webs and rims; PA1 the cover has two lateral edges, retaining means for the lateral edges being disposed on inside faces of the webs of the two uprights; PA1 the sheet has two lateral edges with hems into which two rods are threaded, the rods each having a top end and a bottom end; and PA1 the seat back structure further comprises means for immobilizing the top and bottom ends of the rods which fix each rod in the housing of the respective upright to stretch the sheet in cavity, substantially parallel to the rear face of the seat back, the cover extending over a part of the inside face of each web and over each rim, being pressed into the housings by the rods. PA1 the webs of the lateral uprights extend towards the rear face of the seat back and the rim is substantially parallel to the web so that the housings are open towards the front face of the seat back; PA1 the lateral uprights each comprise a lateral flange which includes a web and front and rear rims bent towards the interior of the seat and facing each other; PA1 the lateral uprights each comprise a tube to which is attached a web that extends towards the rear of the seat and which has a rim bent towards the front of the seat; PA1 the retaining means for the lateral edges of the cover comprise hooks facing towards the front face of the seat; PA1 the immobilizing means comprise a bottom crosspiece connecting the lateral uprights and which has two ends, each of the ends comprising a recess substantially facing each of the housings and in which the bottom end of each of the rods engages; PA1 the immobilizing means further comprise a top wire fixed to the lateral uprights and on which the top ends of the rods abut; PA1 the rods are elastically flexible in the direction towards the rear face of the seat; and PA1 the bottom end of each rod is curved towards the front of the seat in order to engage elastically in the corresponding housing.
This prior art has the drawback of dissociating the step of tensioning the cover over the upholstery from the step Of fitting the sheet into the cavity. This succession of steps is costly in terms of labor and requires a seat back structure of complex shape.